Probing polyvalency in artificial systems exhibiting molecular recognition

An approach to the study of polyvalency-the interaction of polyvalent receptors with polyvalent ligands-in unnatural systems is outlined. In this study, the complexation of dibenzylammonium cations by dibenzo[24]crown-8 or benzometaphenylene[25]crown-8 is utilized as the component receptor-ligand interaction. Two analogous multivalent receptors-each containing either seven dibenzo[24]crown-8 (DB24C8 CLUSTER) or seven benzometaphenylene[25]crown-8 (BMP25C8 CLUSTER) moieties appended to a modified beta-cyclodextrin core-were prepared in moderate yields. For each of these multivalent receptors, complementary mono- and divalent ligands containing one or two dialkylammonium centers, respectively, were prepared in good yields. These ligands contained fluorine atom substituents to allow their interactions with crown ether compounds to be probed by (19)F NMR spectroscopy. The complexation of these monovalent ligands with the DB24C8 CLUSTER and the BMP25C8 CLUSTER was studied by determining the average binding constant (K(AVE)) between the receptors and ligands. The abilities of the crown ether clusters to complex with these monovalent ligands was compared with those of the monovalent crown ethers dibenzo[24]crown-8 and benzometaphenylene[25]crown-8. In both instances, it was found that clustering seven crown ethers together into one molecule is detrimental to the abilities of the crown ether moieties to complex with monovalent dialkylammonium ligands. The complexation of the divalent ligands by the DB24C8 CLUSTER and the BMP25C8 CLUSTER was then studied-again by determining K(AVE)-and their abilities to complex with these ligands was compared with those of their respective component interactions. By determining K(AVE) for the polyvalent interaction, it was possible to calculate an association constant, K(POLY), for the binding of the divalent ligands by the DB24C8 CLUSTER and the BMP25C8 CLUSTER compounds. In both instances K(POLY) for the polyvalent interaction was found to be approximately 2 orders of magnitude higher than the association constants, K(A), for the component interaction.     

The influence of constitutional isomerism and change on molecular recognition processes

Three constitutionally isomeric bis(naphthylmethyl)ammonium ions, in which the two naphthyl groups are substituted 1) both at their 1-positions, 2) one at its 1-position and the other at its 2-position, and 3) both at their 2-positions, have been investigated separately in solution for their propensities to undergo spontaneous self-assembly with three different [24]crown-8 derivatives, namely, pyrido[24]crown-8 (P24C8), dipyrido[24]crown-8 (DP24C8) and dibenzo[24]crown-8 (DB24C8), in turn to form [2]pseudorotaxanes. The strengths of the 1:1 complexes depend on the composition of the secondary dialkylammonium ions and on the nature of the crown ether hosts; generally, as far as the guest cation is concerned, the 1/1- and 2/2-isomers form stronger complexes, as indicated by stability constant measurements, than the 1/2-isomer and, as far as the crown ethers are concerned, the more flexible P24C8 is a much more efficient host than either DP24C8 or DB24C8. The rates of formation of the [2]pseudorotaxanes are fast (i.e., taking no more than a few minutes) in solution with the exception of one case, that is, in which the crown ether host is DB24C8 and the guest cation is the 1/1-isomer, when it can take upwards of one month for the complexation-decomplexation equilibrium to be established at room temperature. In all cases, the equilibrium between complexed and uncomplexed species is slow on the NMR timescale, allowing the determination of stability constants to be made readily using the single-point method. X-ray crystallography and molecular modeling have been used to gain insight into ground and transition state interactions, respectively, in some of the [2]pseudorotaxanes. The relative stabilities of the three [2]pseudorotaxanes formed by each guest cation in the presence of the three crown ether hosts were also evaluated in solution by competition experiments that were monitored by (1)H NMR spectroscopy. By and large the results of the competition experiments could be predicted on the basis of the derived stability constants for the individual [2]pseudorotaxanes.     

Post-assembly processing of [2]rotaxanes

The concept of using [2]rotaxanes that carry one or more surrogate stoppers which can subsequently be converted chemically into other structural units, resulting in the formation of new interlocked molecular compounds, is introduced and exemplified. Starting from simple NH2(+)-centered/crown-ether-based [2]rotaxanes, containing either one or two benzylic triphenylphosphonium stoppers, the well-known Wittig reaction has been employed to make, 1) other [2]rotaxanes, 2) higher order rotaxanes, 3) branched rotaxanes, and 4) molecular shuttles--all isolated as pure compounds, following catalytic hydrogenations of their carbon-carbon double bonds, obtained when aromatic aldehydes react with the ylides produced when the benzylic triphenylphosphonium derivatives are treated with strong base. The two starting [2]rotaxanes were characterized fully in solution and also in the solid state by X-ray crystallography. The new interlocked molecular compounds that result from carrying out post-assembly Wittig reactions on two [2]rotaxanes were characterized by (dynamic) 1H NMR spectroscopy. In the case of a molecular shuttle in which the crown ether component is dibenzo[24]-crown-8 (DB24C8), shuttling is slow on the 1H NMR timescale, even at high temperatures. However, when DB24C8 is replaced by benzometaphenylene[25]-crown-8 as the ring component in the molecular shuttle, the frequency of the shuttling is observed to be around 100 Hz in [D4]methanol at 63 degrees C.     

Mild and high-yielding syntheses of diethyl phosphoramidate-stoppered [2]rotaxanes

The mild and efficient reaction between triethyl phosphite and benzylic azides allows us not only to construct rotaxanes in high yield from dibenzo[24]crown-8 (DB24C8) and dibenzylammonium (DBA(+))-derived threads but also to incorporate di(p-toluidine)[24]crown-8, which binds DBA(+) ions much more weakly than does DB24C8, into a corresponding [2]rotaxane.     

Translational isomerism in a [3]catenane and a [3]rotaxane

[structure: see text] Post-assembly covalent modification using Wittig chemistry of [2]rotaxane ylides, wherein NH(2)(+) centers in the dumbbell-shaped components are recognized by dibenzo[24]crown-8 (DB24C8) rings, has afforded a [3]catenane and a [3]rotaxane with a precise and synthetically prescribed shortage of DB24C8 rings. The nondegenerate pairs of translational isomers present in both of these interlocked molecular compounds provide the fundamental platform on which to construct sensory devices and nanochemomechanical systems.     

Operating molecular elevators

Inspired by the concept of multivalency in living systems, two mechanically interlocked molecules have been conceived that incorporate not once or twice but thrice the features of a pH-switchable [2]rotaxane with two orthogonal recognition sites for dibenzo[24]crown-8 (DB24C8), and 2,3-dinaphtho[24]crown-8 (DN24C8)-one a dialkylammonium ion (CH(2)NH(2)(+)CH(2)) and the other a bipyridinium dication (BIPY(2+)). Whereas at low pH, the CH(2)NH(2)(+)CH(2) sites bind the DB24C8/DN24C8 macrocycles preferentially, at high pH, deprotonation occurs with loss of hydrogen bonding and the macrocycles will move to the BIPY(2+) sites, where they can acquire some stabilizing [pi-pi] stacking interactions. Such mechanically interlocked molecules have been assembled from a trifurcated rig-like component wherein the dumbbell-like components of three [2]rotaxanes have one of their ends fused onto alternate positions (1,3,5) around a benzenoid core. The rig is mechanically interlocked by a platform based on a tritopic receptor, wherein either three benzo[24]crown-8 or three 2,3-naphtho[24]crown-8 macrocycles are fused onto a hexaoxatriphenylene core. The synthesis of these molecular elevators involves 1:1 complexation, followed by stoppering, i.e., feet are added to the rig. (1)H NMR spectroscopy and cyclic voltammetry, aided and abetted by absorption spectroscopy, have been employed to unravel the details of the mechanism by which the rig and platform components move on the alternate addition of base and acid. For each molecular elevator, the platform operates by taking three distinct steps associated with each of the three deprotonation/reprotonation processes. Thus, molecular elevators are more reminiscent of a legged animal than they are of passengers on freight elevators.     

Ammonium ion binding with pyridine-containing crown ethers

Dipyrido[24]crown-8 (DP24C8) has been synthesized and shown to form [2]pseudorotaxanes spontaneously with dibenzylammonium ions. These complexes, which have been demonstrated by (1)H NMR spectroscopy to form faster in solution than when the macrocyclic polyether is dibenzo[24]crown-8 (DB24C8), are also stronger than their DB24C8 counterparts. One of the [2]pseudorotaxanes has been used to construct a [2]rotaxane (see above) comprising a dumbbell-shaped component based on a dibenzylammonium ion which is encircled by a DP24C8 macrocycle and terminated by (triphenylphosphonium)methyl stoppers.     

Supramolecular cation assemblies of hydrogen-bonded (NH4+/NH2NH3+)(crown ether) in [Ni(dmit)2]-based molecular conductors and magnets

Hydrogen-bonded supramolecular cation assemblies of (NH4+/NH2-NH3+)(crown ether), where the crown ether is [12]crown-4, [15]crown-5, or [18]crown-6, were incorporated into electrically conducting [Ni(dmit)2] salts (dmit2- = 2-thioxo-1,3-dithiole-4,5-dithiolate). (NH4+)([12]crown-4)[Ni(dmit)2]3(CH3CN)2 had a pyramidal shape, while ionic channels were observed in (NH4+)(0.88)([15]crown-5)[Ni(dmit)(2)]2 and (NH4+)(0.70)([18]crown-6)[Ni(dmit)(2)]2. Both (NH4+)(0.88)([15]crown-5) and (NH4+)(0.70)([18]crown-6) contained regularly spaced [Ni(dmit)(2)] stacks formed by N-H.O hydrogen bonding between the oxygen atoms in crown ethers and the NH4+ ion. NH4+ occurred nonstoichiometrically; there were vacant ionic sites in the ionic channels. The ionic radius of NH4+ is larger than the cavity radius of [15]crown-5 and [18]crown-6. Therefore, NH4+ ions could not pass through the cavity and were distributed randomly in the ionic channels. The static disorder caused the conduction electrons to be randomly localized to the [Ni(dmit)2] stacks. Hydrazinium (NH2-NH3+) formed the supramolecular cations in (NH2-NH3+)([12]crown-4)2[Ni(dmit)2]4 and (NH2-NH3+)2([15]crown-5)3[Ni(dmit)2]6, possessing a sandwich and club-sandwich structure, respectively. To the best of our knowledge, these represent the first hydrazinium-crown ether assemblies to be identified in the solid. In the supramolecular cations, hydrogen bonding was detected between the ammonium or the amino protons of NH2-NH3+ and the oxygen atoms of crown ethers. The sandwich-type cations coexisted with the [Ni(dmit)2] dimer stacks. Although the assemblies were typically semiconducting, ferromagnetic interaction (Weiss temperature = +1 K) was detected in the case of (NH2-NH3+)2([15]crown-5)3[Ni(dmit)2]6. The (NH2-NH3+)0.8([18]crown-6)[Ni(dmit)2]2 and (NH4+)0.76([18]crown-6)[Ni(dmit)2]2 crystals were isomorphous. The large and flexible [18]crown-6 allowed for maintaining the same ionic channel structure through replacement of the NH4+ cation by NH2-NH3+.     

2]Rotaxanes containing pyridinium-phosphonium axles and 24-crown-8 ether wheels

A triethylphosphonium group attached to a pyridinium ethane moiety can be used as an axle for the self-assembly of [2]pseudorotaxanes and [2]rotaxanes. Although [2]pseudorotaxane formation is limited due to the bulk of the PR4+ group, [2]rotaxanes can be formed utilising 24-crown-8 ether, benzo-24-crown-8 ether and naphtho-24-crown-8 ether. The synthesis of these [2]rotaxanes and the X-ray structure of the [2]rotaxane containing a 24-crown-8 ether wheel are described. When the crown ether contains an aromatic group two possible conformational isomers exist; these are identified at low temperature by 1H and 31P NMR spectroscopy.     

Supramolecular daisy chains

Two series of self-complementary daisy chain monomers, in which a secondary ammonium ion-containing arm is grafted onto a macrocycle with either a [24]- or [25]crown-8 constitution, have been synthesized. In the solid- and 'gas'-phases, the parent [24]crown-8-based monomer forms dimeric superstructures, as revealed by X-ray crystallography and mass spectrometry, respectively. Elucidation of the complicated solution-phase behavior of this compound was facilitated by the synthesis and study of both deuterated, and fluorinated, analogues. These investigations revealed that the cyclic dimeric superstructure also dominates in solution, except when extremes of either concentration (low), temperature (high), or solvent polarity (highly polar, e.g., dimethyl sulfoxide) are employed. Whereas, upon aggregation, the [24]crown-8-based daisy chain monomers have the capacity to form stereoisomeric superstructures further complicating the study of this series of compounds. The assembly of [25]crown-8-based monomers gives only achiral superstructures. The weaker association exhibited between secondary dialkylammonium ions and crown ethers with a [25]crown-8 constitution, however, resulted in limited oligomerization--only dimeric and trimeric superstructures were formed at experimentally attainable concentrations--of [25]crown-8-based daisy chain monomers.     

Four‐State Molecular Shuttling of [2]Rotaxanes in Response to Acid/Base and Alkali‐Metal Cation Stimuli

In this study, we synthesized [2]rotaxanes possessing three recognition sites—a dialkylammonium, an alkylarylamine, and a tetra(ethylene glycol) stations—in their dumbbell‐like axle component and dibenzo[24]crown‐8 (DB24C8) as their macrocyclic component. These [2]rotaxanes behaved as four‐state molecular shuttles: i) under acidic conditions, the DB24C8 unit encircled both the dialkylammonium and alkylarylammonium stations; ii) under neutral conditions, the dialkylammonium unit was the predominant station for the DB24C8 component; iii) under basic conditions, when both ammonium centers were deprotonated, the alkylarylamine unit became a suitable station for the DB24C8 component; and iv) under basic conditions in the presence of an alkali‐metal cation, the tetra(ethylene glycol) unit recognized the DB24C8 component through cooperative binding of the alkali‐metal ion. In addition, we observed that the [2]rotaxanes exhibited selective recognition for metal cations. These shuttling motions of the macrocyclic component proceeded reversibly.     

Which One is Bulkier: The 3,5‐Dimethylphenyl or the 2,6‐Dimethylphenyl Group? Development of Size‐Complementary Molecular and Macromolecular [2]Rotaxanes

We developed novel size‐complementary molecular and macromolecular rotaxanes using a 2,6‐dimethylphenyl terminal group as the axle‐end‐cap group in dibenzo‐24‐crown‐8‐ether (DB24C8)‐based rotaxanes, where the 2,6‐dimethylphenyl group was found to be less bulky than the 3,5‐dimethylphenyl group. A series of molecular and macromolecular [2]rotaxanes that bear a 2,6‐dimethylphenyl group as the axle‐end‐cap were synthesized using unsubstituted and fluorine‐substituted DB24C8. Base‐induced decomposition into their constituent components confirmed the occurrence of deslipping, which supports the size‐complementarity of these rotaxanes. The deslipping rate was independent of the axle length but dependent on the DB24C8 substituents. A kinetic study indicated the rate‐determining step was that in which the wheel is getting over the end‐cap group, and deslipping proceeded via a hopping‐over mechanism. Finally, the present deslipping behavior was applied to a stimulus‐degradable polymer as an example for the versatile utility of this concept in the context of stimulus‐responsive materials.     

Synthesis and characterisation of helical beta-peptide architectures that contain (S)-beta3-HDOPA(Crown Ether) derivatives

A new set of beta-amino acids that carry various crown ether receptors on their side chains of the general formula (S)-beta(3)-HDOPA(crown ether) (HDOPA: homo-3,4-dihydroxyphenylalanine; (crown ether): [15]crown-5 ([15-C-5]), [18]crown-6 ([18-C-6]), [21]crown-7 ([21-C-7]), 1,2-Benzo-[24]crown-8 ([Benzo-24-C-8]) and (R)-Binol-[20]crown-6 ([(R)-Binol-20-C-6])) was prepared. Peptides that are based on these new crowned beta-amino acids combined with (1S,2S)-ACHC (2-aminocyclohexanecarboxylic acid), which is known to be a potent 3(14)-helix inducer, to the hexamer level, with two crowned residues at the i and i+3 positions of the main-chain, were synthesized in solution by stepwise coupling using Boc-N(alpha)-protection (Boc: tert-butoxycarbonyl) and the EDC/HOAt C-activation method. Their conformational analysis was performed by using FTIR absorption, NMR and CD spectroscopy techniques. Our results are in full agreement with a 3(14)-helix conformation.     

Three-state molecular shuttles operated using acid/base stimuli with distinct outputs

This paper describes the acid/base-mediated three-state translational isomerization of two [2]rotaxanes, each containing N-alkylaniline and N,N-dialkylamine centers as binding sites for threaded dibenzo[24]crown-8 units. Under neutral conditions, the dialkylamine unit predominantly recognized the crown ether component through cooperative binding of a proton; when both amino units were protonated under acidic conditions, both translational isomers were generated; the addition of a strong base caused aniline-crown ether interactions to dominate. The three states of the [2]rotaxane featuring the 3,5-diphenylaniline terminus in its dumbbell-shaped component were accompanied by distinct absorptive outputs that were detectable using UV spectroscopy.     

Ferrocene-containing [2]- and [3]rotaxanes. Preparation via an end-capping cross-metathesis reaction and electrochemical properties

Cross-metathesis reactions of terminal olefins with acrylic esters catalyzed by a Ru-carbene complex ((H2IMes)(PCy3)Cl2Ru = CHPh, H2IMes = N,N-bis(mesityl)-4,5-dihydroimidazol-2-ylidene) were applied to the end-capping of [2]pseudorotaxanes composed of dibenzo[24]crown-8 (DB24C8) and ferrocenylmethylammonium derivatives as the macrocyclic and axle components. A [3]rotaxane consisting of two DB24C8s and an axle molecule having ferrocenyl groups at both ends was obtained from the cross-metathesis reaction of two [2]pseudorotaxanes with Fe(C5H4CH2OCOCH = CH2)2. Cyclic voltammograms of the ferrocene-containing rotaxanes show reversible redox reactions whose potentials vary depending on the presence or absence of cationic dialkylammonium groups in the vicinity of the ferrocene units.     

Coordination-driven self-assembly of cavity-cored multiple crown ether derivatives and poly[2]pseudorotaxanes

The synthesis of a new 120 degree diplatinum(II) acceptor unit and the self-assembly of a series of two-dimensional metallacyclic polypseudorotaxanes that utilize both metal-ligand and crown ether-dialkylammonium noncovalent interactions are described. Judiciously combining complementary diplatinum(II) acceptors with bispyridyl donor building blocks, with an acceptor and/or donor possessing a pendant dibenzo[24]crown-8 (DB24C8) moiety, allows for the formation of three new rhomboidal bis-DB24C8, one new hexagonal tris-DB24C8, and four new hexakis-DB24C8 metallacyclic polygons in quantitative yields. The size and shape of each assembly, as well as the location and stoichiometry of the DB24C8 macrocycle, can be precisely controlled. Each polygon is able to complex two, three, or six dibenzylammonium ions without disrupting the underlying metallacyclic polygons, thus producing eight different poly[2]pseudorotaxanes and demonstrating the utility and scope of this orthogonal self-assembly technique. The assemblies are characterized with one-dimensional multinuclear ((1)H and (31)P) and two-dimensional ((1)H-(1)H COSY and NOESY) NMR spectroscopy as well as mass spectrometry (ESI-MS). Further analysis of the size and shape of each assembly is obtained through molecular force-field simulations. (1)H NMR titration experiments are used to establish thermodynamic binding constants and poly[2]pseudorotaxane/dibenzylammonium stoichiometries. Factors influencing the efficiency of poly[2]pseudorotaxane formation are discussed.     

Solid-state molecular rotators of anilinium and adamantylammonium in [Ni(dmit)2](-) salts with diverse magnetic properties

Supramolecular rotators of hydrogen-bonding assemblies between anilinium (Ph-NH 3 (+)) or adamantylammonium (AD-NH 3 (+)) and dibenzo[18]crown-6 (DB[18]crown-6) or meso-dicyclohexano[18]crown-6 (DCH[18]crown-6) were introduced into [Ni(dmit) 2] salts (dmit (2-) is 2-thioxo-1,3-dithiole-4,5-dithiolate). The ammonium moieties of Ph-NH 3 (+) and AD-NH 3 (+) cations were interacted through N-H (+) approximately O hydrogen bonding with the six oxygen atoms of crown ethers, forming 1:1 supramolecular rotator-stator structures. X-ray crystal-structure analyses revealed a jackknife-shaped conformation of DB[18]crown-6, in which two benzene rings were twisted along the same direction, in (Ph-NH 3 (+))(DB[18]crown-6)[Ni(dmit) 2] (-) ( 1) and (AD-NH 3 (+))(DB[18]crown-6)[Ni(dmit) 2] (-) ( 3), whereas the conformational flexibility of two dicyclohexyl rings was observed in (Ph-NH 3 (+))(DCH[18]crown-6)[Ni(dmit) 2] (-) ( 2) and (AD-NH 3 (+))(DCH[18]crown-6)[Ni(dmit) 2] (-) ( 4). Sufficient space for the molecular rotation of the adamantyl group was achieved in the crystals of salts 3 and 4, whereas the rotation of the phenyl group in salts 1 and 2 was rather restricted by the nearest neighboring molecules. The rotation of the adamantyl group in salts 3 and 4 was evidenced from the temperature-dependent wide-line (1)H NMR spectra, dielectric properties, and X-ray crystal structure analysis. ab initio calculations showed that the potential energy barriers for the rotations of adamantyl groups in salts 3 (Delta E approximately 18 kJmol (-1)) and 4 (Delta E approximately 15 kJmol (-1)) were similar to those of ethane ( approximately 12 kJmol (-1)) and butane (17-25 kJmol (-1)) around the C-C single bond, which were 1 order of magnitude smaller than those of phenyl groups in salts 1 (Delta E approximately 180 kJmol (-1)) and 2 (Delta E approximately 340 kJmol (-1)). 1D or 2D [Ni(dmit) 2] (-) anion arrangements were observed in the crystals according to the shape of crown ether derivatives. The 2D weak intermolecular interactions between [Ni(dmit) 2] (-) anions in salts 1 and 3 led to Curie-Weiss behavior with weak antiferromagnetic interaction, whereas 1D interactions through lateral sulfur-sulfur atomic contacts between [Ni(dmit) 2] (-) anions were observed in salts 2 and 4, whose magnetic behaviors were dictated by ferromagnetic (salt 2) and singlet-triplet (salt 4) intermolecular magnetic interactions, respectively.     

Neutralization of a sec-ammonium group unusually stabilized by the "rotaxane effect": synthesis, structure, and dynamic nature of a "free" sec-amine/crown ether-type rotaxane

A fifteen-year riddle has been settled: neutralization, the most popular chemical event, of a crown ether/sec-ammonium salt-type rotaxane has been achieved and a completely nonionic crown ether/sec-amine-type rotaxane isolated. A [2]rotaxane was prepared as a typical substrate from a mixture of dibenzo[24]crown-8 ether (DB24C8) and sec-ammonium hexafluorophosphate (PF(6)) with a terminal hydroxy group through end-capping with 3,5-dimethylbenzoic anhydride in the presence of tributylphosphane as a catalyst in 90% yield. A couple of approaches to the neutralization of the ammonium rotaxane were investigated to isolate the free sec-amine-type rotaxane by decreasing the degree of thermodynamic and kinetic stabilities. One approach was the counteranion-exchange method in which the soft counterion PF(6)(-) was replaced with the fluoride anion by mixing with tetrabutylammonium fluoride, thus decreasing the cationic character of the ammonium moiety. Subsequent simple washing with a base allowed us to isolate the free sec-amine-type rotaxane in a quantitative yield. The other approach was a synthesis based on a protection/deprotection protocol. The acylation of the sec-ammonium moiety with 2,2,2-trichloroethyl chloroformate gave an N-carbamated rotaxane that could be deprotected by treating with zinc in acetic acid to afford the corresponding free sec-amine-type rotaxane in a quantitative yield. The structure of the free sec-amine-type rotaxane was fully confirmed by spectral and analytical data. The generality of the counteranion-exchange method was also confirmed through the neutralization of a bisammonium-type [3]rotaxane. The mechanism was studied from the proposed potential-energy diagram of the rotaxanes with special emphasis on the role of the PF(6)(-) counterion.     

Dynamic covalent approach to [2]- and [3]roxtanes by utilizing a reversible thiol-disulfide interchange reaction

A dynamic covalent approach to disulfide-containing [2]- and [3]rotaxanes is described. Symmetrical dumbbell-shaped compounds with two secondary ammonium centers and a central located disulfide bond were synthesized as components of rotaxanes. The rotaxanes were synthesized from the dumbbell-shaped compounds and dibenzo-[24]crown-8 (DB24C8) with catalysis by benzenethiol. The yields of isolated rotaxanes reached about 90 % under optimized conditions. A kinetic study on the reaction forming [2]rotaxane 2 a and [3]rotaxane 3 a suggested a plausible reaction mechanism comprising several steps, including 1) initiation, 2) [2]rotaxane formation, and 3) [3]rotaxane formation. The whole reaction was found to be reversible in the presence of thiols, and thermodynamic control over product distribution was thus possible by varying the temperature, solvent, initial ratio of substrates, and concentration. The steric bulk of the end-capping groups had almost no influence on rotaxane yields, but the structure of the thiol was crucial for reaction rates. Amines and phosphines were also effective as catalysts. The structural characterization of the rotaxanes included an X-ray crystallographic study on [3]rotaxane 3 a.     

A versatile template for the formation of [2]pseudorotaxanes. 1,2-Bis(pyridinium)ethane axles and 24-crown-8 ether wheels

Linear 1,2-bis(pyridinium)ethane 'axles' and macrocyclic 24-membered crown ether 'wheels' (, and ) combine to form [2]pseudorotaxanes. These interpenetrated adducts are held together by N+...O ion-dipole interactions, a series of C-H...O hydrogen bonds and pi-stacking between electron-poor pyridinium rings of the axle and electron-rich catechol rings of the wheel. 1H NMR spectroscopy was used to identify the structural details of the interaction and to determine the thermodynamics of the binding process in solution. Analysis of nine of these adducts by single crystal X-ray crystallography allowed a detailed study of the non-covalent interactions in the solid state. A wide variety of structural changes could be made to the system. The versatility and potential of the template for the construction of permanently interlocked structures such as rotaxanes and catenanes is discussed.